1. Field of the Invention
This invention relates to thermally conductive silicone rubber compositions, and, more particularly, to thermally conductive silicone rubber compositions in which the fine alumina powder does not readily precipitate and in which the fine alumina powder can easily be uniformly redispersed if it does precipitate.
2. Description of the Related Art
As electronic devices, such as printed circuit boards and hybrid integrated circuits, that carry electronic components such as transistors, integrated circuits and memory elements have become more dense and more highly integrated in recent years, various types of thermally conductive silicone rubber compositions have been used to efficiently dissipate heat from the devices. The thermally conductive silicone rubber compositions that have been used include thermally conductive silicone rubber compositions that have been hardened by addition reactions, such as thermally conductive silicone rubber compositions that contain adhesion promoters and are comprised of organopolysiloxanes containing vinyl groups, organohydrogen polysiloxanes, fine alumina powders, an adhesion promoter and a platinum catalyst (see Japanese Patent Application Laid Open No. 61-157569 (1986)), thermally conductive silicone rubber compositions that are comprised of organopolysiloxanes containing at least 0.1 mol % of alkeny groups in the molecule, organohydrogen polysiloxanes containing at least two hydrogen atoms bonded with silicon atoms in the molecule, spherical fine alumina powder of an average particle diameter of 10 to 50 .mu.m, spherical or nonspherical fine alumina powder of an average particle diameter of up to 10 .mu.m and platinum or a platinum compound (see Japanese Patent Application Laid Open No. 63-251466 (1988)), thermal conductive silicone rubber compositions that are comprised of organopolysiloxanes containing alkenyl groups, organohydrogen polysiloxanes, amorphous fine alumina powder of an average particle diameter of 0.1 to 5 .mu.m, spherical fine alumina powder of an average particle diameter of 5 to 50 .mu.m and platinum catalysts (see Japanese Patent Application Laid Open No. 241362 (1990)) and thermally conductive silicone rubber compositions that are comprised of organopolysiloxanes containing more than an average of 0.5 alkenyl groups in the molecule, organohydrogen polysiloxane containing at least two hydrogen atoms bonded with silicon atoms in the molecule, high purity fine alumina powder of which the average particle diameter is less than 50 .mu.m and of which the major axis-minor axis ratio was 1.0 to 1.4 and a platinum catalyst (see Japanese Patent Application Laid Open No. 5-105814 (1993)).
In order to increase the coefficient of thermal conductivity of silicone rubber that is obtained by hardening this type of thermally conductive silicone rubber composition, fine alumina powder is generally compounded in large quantities in this composition. However, because the fine alumina powder has extremely high specific gravity, it readily precipitates in the silicone rubber composition. A further problem is that it is not easy to redisperse fine alumina powder that has precipitated from the composition.
The inventors conducted intensive studies for the purpose of solving the aforementioned problems, and, as a result, they arrived at this invention.
Specifically, the objective of this invention is to provide a thermally conductive silicone rubber composition in which, in spite of the fact that a large quantity of fine alumina powder is compounded in order to form silicone rubber of high thermal conductivity, the fine alumina powder does not readily precipitate and in which the fine alumina powder can easily be uniformly redispersed even if it does precipitate.